1. Field of the Invention:
This invention relates to a collimator lens which is useful to an optical system for reading from or writing in a recording medium.
2. Description of the Prior Art:
Recently, information processing using a high-density recording medium such as a compact disc, an optical disc or the like, has been rapidly advanced, in which an optical system is generally employed for reading from or writing in the recording medium.
In such an optical system, diffused light from a light source such as a semiconductor laser is converted to parallel rays by a collimator lens and then converged onto the surface of a recording medium by an objective lens. In addition to such an optical reading or writing system, a collimator lens has been generally and widely used for the purpose that diffused light is converted to parallel rays. When a collimator lens having reduced spherical aberration and coma is required, a compound lens consisting of plural spherical lenses has been used. In that case, however, it is hard to reduce the cost of the system.
In order to solve the above problem of such a compound lens system, system using an aspherical lens or a gradient index lens having refractive index gradient in the radial direction has been developed. Aspherical lenses are, however, unsuitable for mass-production because the aspherical surface is difficult to process or take an accurate measurement. On the other hand, a radial gradient index lens is made by an ion-exchange process of glass, however, the ion-exchange process has need of a long process time so that only a small lens having the effective aperture less than 5 mm can be obtained in practice.